Add statement continuation in REPL in CLI

Typing ```
var foo = [
``` into the REPL now causes it to prompt you to complete the
statement rather than just printing an "unexpected EOF" error.

ablescript/src/lib.rs

@@ -1,12 +1,12 @@
 #![forbid(unsafe_code, clippy::unwrap_used)]
 
 pub mod ast;
+pub mod error;
 pub mod interpret;
 pub mod parser;
 
 mod base_55;
 mod brian;
 mod consts;
-mod error;
 mod lexer;
 mod variables;

ablescript_cli/src/repl.rs

@@ -6,39 +6,44 @@ use ablescript::parser::Parser;
 pub fn repl(ast_print: bool) {
     let mut rl = Editor::<()>::new();
     let mut env = ExecEnv::new();
+
+    // If this is `Some`, the user has previously entered an
+    // incomplete statement and is now completing it; otherwise, the
+    // user is entering a completely new statement.
+    let mut partial: Option<String> = None;
     loop {
-        let readline = rl.readline(":: ");
-        match readline {
-            Ok(line) => {
-                // NOTE(Alex): `readline()` leaves a newline at the
-                // end of the string if stdin is connected to a file
-                // or unsupported terminal; this can interfere with
-                // error printing.
-                rl.add_history_entry(&line);
-                let line = line.trim_end();
+        match rl.readline(if partial.is_some() { ">> " } else { ":: " }) {
+            Ok(readline) => {
+                let readline = readline.trim_end();
+                let partial_data = match partial {
+                    Some(line) => line + &readline,
+                    None => readline.to_owned(),
+                };
 
-                if line == "exit" {
-                    println!("bye");
-                    break;
-                }
-
-                let mut parser = Parser::new(line);
-                let value = parser.init().and_then(|ast| {
+                partial = match Parser::new(&partial_data).init().and_then(|ast| {
                     if ast_print {
                         println!("{:#?}", &ast);
                     }
                     env.eval_stmts(&ast)
-                });
-
-                if let Err(e) = value {
+                }) {
+                    Ok(_) => None,
+                    Err(ablescript::error::Error {
+                        // Treat "Unexpected EOF" errors as "we need
+                        // more data".
+                        kind: ablescript::error::ErrorKind::UnexpectedEof,
+                        ..
+                    }) => Some(partial_data),
+                    Err(e) => {
                         println!("{}", e);
-                    println!(" | {}", line);
+                        println!(" | {}", partial_data);
                         println!(
                             "   {}{}",
                             " ".repeat(e.span.start),
                             "^".repeat((e.span.end - e.span.start).max(1))
                         );
+                        None
                     }
+                };
             }
             Err(rustyline::error::ReadlineError::Eof) => {
                 println!("bye");